There has been known a surface acoustic wave device, which includes a comb-shaped electrode formed of IDTs (Interdigital Transducer) formed on a surface of a piezoelectric substrate, an FBAR (Film Bulk Acoustic Resonator) device, in which a lower electrode, a piezoelectric film, and an upper electrode are stacked on a substrate, and a Lamb wave device, in which a lower electrode, a piezoelectric film, and an upper comb-shaped electrode are stacked, as acoustic wave devices using acoustic waves. These acoustic wave devices are small and light, can obtain high attenuation, and thus are used as filters of wireless devices such as mobile phone terminals.
Recent advancements in high performance of mobile phone terminals require acoustic wave devices to improve temperature characteristics and to reduce the absolute value of a temperature coefficient of frequency (TCF) such as the passband of a filter or the resonance frequency of a resonator. For example, there has been known that the absolute value of the TCF can be made small in the surface acoustic wave device by forming a silicon oxide film having a temperature coefficient opposite in sign to that of the piezoelectric substrate such as LiTaO3(LT) or LiNbO3(LN) on the piezoelectric substrate. There has been known that the TCF can also be made small in the FBAR device by providing a silicon oxide film in a region in which the lower electrode and the upper electrode face each other as disclosed in K. M Lakin and three others, “Temperature Coefficient and Ageing of BAW Composite Materials”, TFR Technologies, Inc. In addition, there has been known that the TCF can also be made small in the Lamb wave device by providing a silicon oxide film below the lower electrode as disclosed in Chin-Ming Lin and five others, “Thermally compensated aluminum nitride Lamb wave resonators for high temperature applications”, APPLIED PHYSICS LETTERS 97, 083501 (2010).
Moreover, there has been known using a silicon oxide film doped with B (boron) as a temperature compensation layer in the FBAR device as disclosed in Japanese Patent Application Publication No. 2007-159123 (Patent Document 1).
Patent Document 1 uses the silicon oxide film doped with B as the temperature compensation layer, but has a problem that even the use of the silicon oxide film doped with B fails to improve a temperature dependence of frequency sufficiently.